Diaphragmatic shear modulus at various submaximal inspiratory mouth pressure levels.

This study assessed the shear modulus of the diaphragm at various submaximal inspiratory mouth pressure levels by ultrasound shear wave elastography. In 14 healthy male subjects, diaphragmatic shear moduli were assessed at end expiration during resting breathing and at submaximal inspiratory tasks at 15, 30, 45, 60, and 75% of the maximal inspiratory mouth pressure. The shear modulus increased along with mouth pressure, and the mouth-pressure-shear-modulus relationship fit the second-order polynomial regression equation (r2 = 0.99 ± 0.01; all subjects, r2 ≥ 0.95) better than it did the simple linear regression equation (r2 = 0.94 ± 0.05; 8/14 subjects, r2 ≥ 0.95). The second regression coefficient in the second-order polynomial equation was a negative value in 10 of 14 subjects, which indicates that the second-order polynomial regression equation opened downwards. These findings suggest that the diaphragmatic shear modulus increases along with inspiratory mouth pressure, but the rate of increase slows when the pressure reaches higher levels.